1. Field of the Invention
The invention relates to wholly aromatic polyesters resins. More specifically, the invention relates to 2,6-naphthalenedicarboxylic acid--4,4'-dihydroxybiphenyl polyesters which have been modified with p-hydroxybenzoic acid. These polyesters are further modified by including terephthalic acid to lower the melting point of the polyester. 2. Discussion of the Background
Liquid crystalline polyesters are known in the art. The polyesters may be either aliphatic-aromatic polyesters such as, for example, polyethylene terephthalate modified with p-hydroxybenzoic acid, or the polyesters may be based on wholly aromatic polyesters such as those prepared from terephthalic acid and biphenol which may be optionally modified with p-hydroxybenzoic acid. A terephthalic acid/4,4-dihydroxybiphenyl/p-hydroxybenzoic acid polyester is available commercially under the Xydar.RTM. trademark. Many of the aromatic polyesters are suitable for high use temperature applications. However, some suffer from the deficiency that melt processing temperatures are generally very high. Xydar, for example, has a melting point of 421.degree. C.
U.S. Pat. No. 3,637,595 discloses wholly aromatic polyesters prepared from 4,4'-dihydroxybiphenyl, such as for example hydroquinone or 4,4'-dihydroxybiphenyl, an aromatic acid and p-hydroxybenzoic acid. According to this patent, the polyesters are prepared in an inert heat transfer liquid such as Therminol or a terphenyl mixture. Melting points of the polymers are very high for polyesters where the diacid is terephthalic acid. Typical softening points for terephthalic acid/ hydroquinone/-p-hydroxybenzoic acid copolyesters presented in the examples are 432.degree.-493.degree. C. An example using 4,4'-dihydroxybiphenyl in place of hydroquinone gave a melting point of 421.degree. C. The single figure exhibits a sharp eutectic point in the melting behavior of the hydroquinone-terephthalic acid polyester. Similar eutectic behavior with lower melting points is predicted when isophthalic acid is substituted for terephthalic acid.
U.S. Pat. No. 4,067,852 discloses wholly aromatic polyesters prepared from 2,6-naphthalenedicarboxylic acid, p-hydroxybenzoic acid and a 4,4'-dihydroxybiphenyl. The melting points of these compositions are lower than those of the compositions prepared in U.S. Pat. No. 3,637,595. For example, a 2,6-naphthalenedicarboxylic acid/4,4'-dihydroxybiphenyl polyester modified with 75 mole percent of p-hydroxybenzoic acid is reported to give a melting point of 395.degree. C. by differential scanning calorimetry (DSC). A 2,6-naphthalenedicarboxylic acid-hydroquinone polyester modified with 75 mole percent of p-hydroxybenzoic acid gave a melting point of 340.degree. C. The polyesters were again prepared in an inert heat transfer fluid. In a typical example, the reaction mixture is heated for 7 hours starting at 200.degree. C. and gradually increasing to 260.degree. C. The temperature is then held constant at 260.degree. C. for 15 hours, followed by stepped increases to 335.degree. C. over a period of 5 hours and then held at 335.degree. C. for an additional hour. These reaction times are excessive and uneconomical from a manufacturing point of view.
U.S. Pat. No. 4,169,933 discloses wholly aromatic polyesters derived from terephthalic acid, 2,6-naphthalenedicarboxylic acid, a diester of hydroquinone and p-acetoxybenzoic acid. The specific polyesters disclosed have melting points in the range of approximately 320.degree. to 405.degree. C. However, the patent fails to disclose or even refer to aromatic polyesters wherein the diol component consists or consists primarily of 4,4'-dihydroxybiphenyl.
U.S. Pat. No. 4,639,504 discloses a broad group of wholly aromatic polyesters in the context of a process of preparing aromatic polyesters having a low degree of coloration and excellent heat stability. The polyesters exemplified are derived solely from terephthalic acid, 4,4'-bisacetoxybiphenyl and p-acetoxybenzoic acid and had melting points in the range of 410.degree. to 421.degree. C. This patent does not suggest that the polyesters provided in accordance with our invention would have melting points in the range of 280.degree. to 350.degree. C.
A continuing need exists, therefore, for wholly aromatic polyesters which are melt processable at relatively low temperatures and yet which retain the desired properties of high tensile strength, flexural strength and impact strength.